This invention relates to a unit for measuring the alkali metal vapor concentration of a gaseous phase, particularly, a unit suitable for measuring the sodium vapor concentration of the gas serving to cover a liquefied metal sodium or the like used as a coolant or heating medium in, for example, a fast breeder.
A liquefied metal is used as a coolant or heating medium in various industrial facilities including a fast breeder. It is customary to use alkali metals like sodium, potassium and alloys thereof as the coolant or heating medium. In some cases, it is necessary to measure the alkali metal concentration of the gas like Ar gas serving to cover the liquefied alkali metal because, if the alkali metal concentration of the cover gas is unduly high, the alkali metal tends to deposit on the piping of the cover gas, thereby causing plugging or corrosion of the cover gas piping. The measurement of alkali metal concentration is also required in some other industrial or laboratory devices handling alkali metals for the purpose of, for example, monitoring the leakage of alkali metal vapor. Among the alkali metals, sodium is most frequently used in the liquedfied form and it is of high importance in many cases to measure the sodium vapor contration.
The prior arts for measuring the sodium vapor concentration include, for example, a sampling method in which a sample gas containing sodium vapor is taken from a sodium vapor system and subjected to a quantitative analysis. However, the sampling method is not satisfactory in that it is impossible to measure continuously the sodium vapor concentration of the gas.
An electric discharge method is also known to the art, which utilizes the fact that the electric discharge initiation voltage between a pair of flat electrodes disposed in parallel in a gas which contains sodium vapor depends on the sodium concentration of the gas. In this method, however, not only the sodium vapor concentration but also the particle size and particle number density of the sodium mist accompanying the sodium vapor provide the parameters of the measured value, resulting in that the pulse signal generated by the discharge fails to reflect accurately the total sodium concentration, i.e., the sum of the sodium vapor and sodium mist. In other words, the conventional electric discharge method is not satisfactory in reliability of the measured value.
As a device capable of overcoming the drawbacks described above and of detecting the sodium vapor concentration at a high accuracy, Japanese Patent publication No. 49-7198/1974 discloses a sodium vapor sensor utilizing a .beta.-alumina porcelain as the electrolyte. The sodium vapor sensor consists essentially of a .beta.-alumina porcelain acting as a partition wall and having one face kept in contact with the gas to be measured, a pair of electrodes mounted such that the .beta.-alumina porcelain is sandwiched therebetween, and an outer circuit connected to the electrodes. In this sensor, the sodium vapor concentration is measured by detecting the ion current caused by the flow of sodium ions formed when sodium vapor contacts the .beta.-alumina porcelain. Alternatively, the electromotive force generated by the difference in sodium vapor pressure between one side and the other side of the .beta.-alumina porcelain is detected for measuring the sodium vapor concentration. Specifically, the difference in vapor pressure mentioned causes the sodium vapor sensor to act as a concentration cell and the electromotive force of the concentration cell is detected for measuring the sodium vapor concentration. Certainly, the sodium vapor sensor disclosed in the Japanese Patent Publication quoted above is advantageous in many respects, but leaves room for further improvement. In general, the sodium vapor introduced into the sensing system contains sodium mist. However, the sensor in question permits detecting the sodium in the form of vapor alone, failing to detect the sodium in the form of mist. It follows that it is impossible to detect the total amount of sodium contained in the gas under examination.
The difficulty mentioned above may be overcome by providing the sensor with a means for heating the .beta.-alumina porcelain to a temperature high enough to vaporize the sodium mist. Certainly, the sensor provided with such a heating means permits detecting all the sodium contained in the gas at high accuracy, but difficulties reside in the field of application of the sensor itself. For example, it is difficult to mount a sensor directly to such large equipment as a pressure vessel of a fast breeder because great restrictions are given by the location, shape, etc. of the portion at which the sensor is mounted. Further, it is difficult to mount the sensor directly to a vessel housing a pressurized hot gas in view of the resistances of the sensor to pressure and heat.